Speaking to reporters on Air Force One on Friday, President Trump said he plans to “look into” reports his national security adviser discussed U.S. sanctions with Russian officials before his inauguration.

The Washington Post, citing current and former officials, reported Thursday night that national security adviser Michael Flynn and Russian ambassador Sergey Kislyak communicated about the sanctions imposed on Russia when President Obama was still in office. The references were interpreted by some officials as "inappropriate" and "potentially illegal," the Post reported.

The report puts pressure on Trump's administration and contradicts Flynn's past denials that communication took place, as well as Vice President Pence's comments made during an interview with CBS News last month. The "strictly coincidental" conversation, the vice president insisted, had "nothing whatsoever to do" with sanctions or the expulsion of Russian diplomats, Pence told Face the Nation host John Dickerson last month.

According to the Associated Press, a Trump administration official said Flynn “can’t be certain” sanctions were brought up during the call. The official added the retired Lt. Gen. had “no recollection” of discussing the sanctions, but left open the possibility that the issue did come up when Flynn spoke with Kislyak during the transition.

The Kremlin denied Friday that Flynn and Kislyak discussed the sanctions before Trump took the Oval Office, the Associated Press reported.

Russian interference in the 2016 presidential election campaign dogged Trump throughout his election and transition, as intelligence agencies delivered increasingly urgent assessments of Russian efforts to hack emails belonging to the Democratic National Committee and advisers of Trump rival Hillary Clinton.

Last week, ranking members of six House committees sent a letter to Defense Secretary Jim Mattis asking him to investigate whether Flynn violated the emoluments clause of the Constitution by receiving payment from a state-sponsored Russian news outlet. Flynn appeared on Russia Today and was paid to speak at a Moscow gala hosted by the network in 2015 — a dinner also attended by Russian President Vladimir Putin.

The emoluments clause prohibits any federal officer from receiving payment for a foreign country without approval by Congress. In an interview with The Post last year, Flynn compared Russia Today to independent U.S. networks CNN or MSNBC and described the RT speech in Moscow as "a paid speaking opportunity" arranged through a speaker's bureau.

Congressional Democrats called for an investigation into Flynn on Friday, demanding Trump fire the retired Lt. Gen. “He lied — repeatedly and egregiously — about his actions,” Rep. Ruben Gallego, D-Ariz., and Ted Lieu, D-Calif., said, according to AP.